Heretofore, Advanced communications today use the Orthogonal Frequency Division Multiplex (OFDM) modulation for efficient transmission of digital signals. These signals may include video, voice and/or data.
OFDM is a commonly used implementation of Multi-Carrier Modulation (MCM).
The Orthogonal Frequency Division Multiplex (OFDM) is a modern advanced modulation method, that achieves better use of the frequency spectrum, as detailed below.
OFDM has been used in recent years in many applications where robustness against severe multipath and interference conditions is required, together with high system capacity, flexibility in providing variable bit rate services, scalability and a capability to perform well in Single Frequency Networks (SNF). OFDM forms the basis for various communication standards, including for example the Digital Terrestrial Television Broadcasting, Digital Audio Broadcasting (DAB), wireless LANs and Wireless Local Loops.
A particular example of the use of OFDM is in DVB systems.
Digital video broadcasting (DVB) systems are now being developed, based on several standards, in Europe, U.S.A. and Japan. Each standard addresses cable broadcasting as well as satellite and terrestrial/area broadcasting.
In Europe, various standards define the digital video broadcasting (DVB) system, including DVB-T (DVB-Terrestrial), DVB-C (DVB-Cable) and DVB-S (DVB-Satellite).
For example, the European standard EN 300 744 defines the DVB-T.
The OFDM modulation method has been chosen, for example, for the digital television broadcasting (MPEG-2) per standard DVB-T.
A disadvantage of presently used DVB-T systems is their unidirectional operation. That is, information is only transmitted from a base station (the transmitter) to subscribers (the receivers). A system may contain many base stations and subscribers, however there is always an unidirectional flow of information.
An interactive system can be achieved by using a telephone line as a return channel from the set top box, however this method is slow and inconvenient.
A fast, flexible link from subscriber to the base station is required for the multitude of advanced services that are in demand today.
To achieve a bi-directional link is a difficult task, that requires an innovative approach as detailed below. The problem is further aggravated by the requirement that the solution should cause no deterioration in performance; moreover, updated subscriber units capable of bi-directional operation should coexist with older subscriber units that do not have these transmission capabilities.
To understand the difficulty of complying with these requirements, one should delve into the intricacies of contemporary digital video broadcasting standards. These standards specify advanced signal processing, to achieve higher quality communications at very high bit rates.
Thus, in the Orthogonal Frequency Division Multiplex (OFDM) modulation method, a block of information is divided among N frequency channels, so that a portion of the information is transmitted in each of the abovementioned channels or frequencies. Since each channel is orthogonal to the others, a better utilization of the frequency spectrum is achieved.
The OFDM method achieves lower Inter-Symbol Interference ISI, since the distribution of the information over N carriers allows each bit of information to be sent for a longer time period (N times longer). For a low ISI, the overlap between adjacent symbols should be lower than 10%. The ISI increases as the percentage of overlap between adjacent symbols increases. In OFDM, since each symbol is N times longer, the percent overlap between adjacent symbols decreases, hence the Inter-Symbol Interference ISI is lower.
Still better spectrum utilization is achieved by QAM (Quadrature Amplitude Modulation) on each of the N carriers.
An IFFT (Inverse Fourier Transform) is performed on the modulated carriers, to form the signal in the time domain that corresponds to the above modulated carriers. The signal is transmitted as a frame that contains the block of information to be transmitted.
A possible problem with the above modulation method is multipath, that may result in interference between adjacent transmitted frames. To address this problem, a guard time period is inserted between adjacent frames. The guard time is especially important in QAM, that is more sensitive to interference. In DVB-T systems, the guard time is chosen as either ¼, ⅛, 1/16 or 1/32 of the symbol time.
A disadvantage of presently used OFDM channels is the need to reserve a guard interval in order to battle multipath and to enable operation in SFN networks. The guard interval, which is up to 25% of the symbol duration, is in effect a wasted time, because no information is transmitted during that time interval.
Although the guard time is used to address the multipath problem, it is a costly solution, since it reduces the capacity of the communication system. It would be highly desirable to use other means for solving the multipath problem, that would allow channel operation at full speed.
Therefore, it is a formidably difficult task to try and improve or change these complex communications standards.
Several methods are now used to separate signals transmitted over a common channel, including:
TDMA—Time Division Multiple Access
FDMA—Frequency Division Multiple Access
DS-CDMA—Direct Sequence/Code Division Multiple Access
CDMA systems may use either DS/CDMA or FH/CDMA. In DS/CDMA multicode or DS/Multicode/CDMA, the separation of signals transmitted over a common channel is achieved using orthogonal codes.
At present, a problem in DS/CDMA is how to generate these orthogonal codes. It is possible to have N channels using orthogonal Walsh codes to multiply each channel, wherein each user has a different Walsh code. In the downlink channel (DL), that is the channel from the base station to subscribers, the orthogonality is preserved, since transmission to all users is prepared and transmitted at the same time. Each user receives all the encoded messages at the same time.
In the uplink, however, each user has a different timing because of a different propagation time delay.
Thus, each Walsh code (corresponding to a specific user) may be shifted in time relative to the other codes (that correspond to the other users). This effect creates interference between channels.
The problem is further aggravated by multipath, that may cause the phase shift in each channel to change in time.
In prior art CDMA, alignment up to a portion of one chip was required to maintain orthogonality between signals. This is a severe requirement, that affects the cost and complexity of the communication equipment.
A reduced orthogonality may cause a higher level of inter-user interference, caused by cross-correlation effects.
Undesired frequency deviations pose a difficult problem in the uplink channel (from subscribers to base), since the base has to concurrently receive and process signals from a plurality of subscribers: each subscriber may have a different deviation, from Doppler Or other causes.
If part of the signals from one user overlap with another's because of a frequency deviation, these signals cannot be separated in the receiver—the damage cannot be repaired, and it may cause interference between users.
Various attempts in prior art at solving this problem are not effective.
In one prior art system, there are a plurality of receivers at the base station, each receiver is tuned to one subscriber transmitter, with a closed loop to track and correct frequency deviations in that transmitter.
Such an approach may be highly complex and expensive when there are a large number of subscriber units.
The above solution cannot be used where subscribers share a common channel, rather frequency multiplexing is used, a different band for each subscriber.
The present disclosure presents a more effective approach, which allows higher performance and flexibility at a lower cost, using a common receiver at the base, for processing signals from all the subscriber transmitters.
In this approach, however, performance degrades rapidly if there are frequency deviations in the received signals. As frequency variations may have different values for the various subscribers, the common receiver cannot track them.
Wideband Systems, and more so broadband systems, are very sensitive to such frequency deviations, which may cause a deterioration in the orthogonality of subcarriers and information in the received signals, resulting in errors, thus reducing channel quality.
A Doppler frequency shift may result from a mobile subscriber's motion. For example, at a frequency of 2 GHz and a vehicle velocity of 100 km/h, a Doppler of about 185 Hz is expected. The received signal may have any value of deviation, between −185 Hz and −185 Hz; the deviation will usually change with time; the same applies to the other subscribers.
At a higher velocity and/or frequency, a larger Doppler deviation is expected (the Doppler deviation is proportional to velocity and frequency). There are other sources of undesired frequency deviations, such as multiplexers in a fiber-optic channel, etc.
In another prior art system, the subscriber locks on a signal from the base and transmits at that signal. One or more pilots may be used. This, however, will not correct for frequency deviations due to multiplexers in the charnel, both in the downlink and the uplink: as the signal from base deviates because of various causes, the signal received back at the base accumulates all the deviations on the way.
This system will not solve the Doppler frequency shift: assuming a moving subscriber receiving a signal from the base at a frequency deviation of +Fd, if it transmits at the received frequency, then the signal received at the base will have double the deviation or +2 Fd, since the Doppler acts the same way in both directions.
Another problem in prior art uplink channels is the need for signals from all the subscribers to arrive at about the same time window at the base, to allow their processing together. Due to different distances to base, the subscribers will be received each at a different time delay—a subscriber farther from base will have a longer time delay.
Yet another problem in prior art communications is the need for dynamic allocation of channels: The radio spectrum is a precious resource; allocating it wisely is of paramount importance.
Various users need each a different channel capacity—some say want to send or receive pictures, which require large amounts of data; others transmit voice or music, which requires less data; others are idle, thinking or waiting for something. Each user's needs may change at a moment's notice. It would be highly desirable to provide a system and method for allocating bandwidth in the amount required by each user—to each user to allocate bandwidth in the system, dynamically, according to their momentary needs.
Prior art systems apparently do not disclose a system similar to that detailed in the present disclosure.
Thus, Seki et al., U.S. Pat. No. 5,771,224, discloses an orthogonal frequency division multiplexing transmission system and transmitter and receiver therefor. It transmits an OFDM transmission frame, with null symbols and reference symbols being placed in the beginning portion of the frame and QPSK symbols are placed in an information symbol data region in the frame, with equal spacing in time and frequency.
The carrier amplitude and phase errors are corrected by a correction information producing section on the amplitude and phase variations of the received signal detected by the variation detector to produce corrected information.
Apparently, Seki does not address the problem of reverse link transmissions. Moreover, Seki performs a different type of signal processing.
Baum et al., U.S. Pat. No. 5,802,044, discloses a multicarrier reverse link timing synchronization system. A center station transmits a forward link signal, receives a reverse link signal, and determines a timing offset for signals received on a reverse link timing synchronization channel. A reverse link symbol timing synchronization can be used in a system having a plurality of transmitting overlap bandwidth subscriber units on an OFDM-like spectrally overlapping reverse channel. The modulation method may comprise M-ary Quadrature Phase Shift Keying(M-PSK), M-ary Quadrature Amplitude Modulation (QAM) or other digital modulation method.
Gudmundson et al., U.S. Pat. No. 5,790,516, discloses a method and system for pulse shaping for data transmission in an orthogonal frequency division multiplexed (OFDM) system.